In electron gun assemblies used, for example, in inductive output tubes (IOTs) and klystrons, it is necessary for optimum performance to ensure that the component electrodes are precisely located relative to one another and are maintained in alignment throughout operation of the device. The electrodes must be aligned both in the longitudinal axial direction, that is, the direction of the electron beam path and also in the transverse direction.
As the temperature of a device changes when it is brought into operation, its dimensions also alter according to the coefficients of thermal expansion of the materials of which it is composed. Different materials may have widely differing thermal expansivities, leading to stresses within the device. In an electron gun, this can lead to distortion and even permanent damage to the structure, particularly where components are fragile and/or must be accurately positioned to very high tolerances.
The problem is particularly acute in high power tubes such as klystrons and IOTs, especially where a gridded electron gun is required in which spacing between the grid and cathode is critical.